1. Field of the Invention
The subject invention relates to electronic devices capable of operating, selectively, from a battery or an AC source.
2. Description of Prior Art
In a known radio receiver design an electrical plug and cord set is used to couple AC power to a DC power supply circuit used to supply power to electronic radio circuits of the receiver. The output of the power supply is connected to a terminal of an ON-OFF switch, which controls the ON-OFF state of the radio circuits, and to a diode connected in series with a battery. The output voltage of the power supply, with respect to the battery, and the orientation of the diode are selected to reverse bias the diode when the power supply is drawing power from an AC source. As a result, in this mode of operation no current is drawn from the battery. Alternatively, if the plug is not connected to an AC source or if the AC source fails, the battery can supply power to the radio circuits. In the battery mode of operation, the maximum voltage available to the radio circuits is equal to the voltage supplied by the battery less the forward bias voltage drop across the diode. To use the available battery energy efficiently it is desirable to have as small a voltage drop as is possible across the diode. Since germanium diodes or germanium transistors connected to function as diodes have a cut-in voltage of 0.2 volts and corresponding silicon devices have a cut-in voltage of about 0.6 volts, the use of a germanium device in the described circuit is preferred. The preference, however, is under pressure because the price of germanium devices is increasing rapidly and availability is decreasing.